Perhaps the most important security feature on your Mac is FileVault. It encrypts your entire hard drive so that even if your Mac gets stolen and the thief has access to it all, they won't be able to access your files without knowing your password.
A feature like this is great if you travel with your Mac frequently and are in situations where your Mac could get stolen.
However, be aware that if you forget your password, there’s absolutely no way to get access to your files without completely reinstalling OS X and starting from scratch. Plus, if you don’t have your Mac backed up, you’ll lose all of your files when you go to reinstall.
Furthermore, encryption itself takes up space, which means turning on FileVault will take up more space on your Mac than without it on. It’s not a lot of space, but you’re already hurting for storage space, this is something to at least keep in mind.